This invention is related to the following application which is assigned to the assignee of the present invention, has a common inventor, and is being filed concurrently: U.S. patent application Ser. No. 09/193,752 (Qtera 2), entitled xe2x80x9cHigh Speed Polarization Independent Modulators Using Lithium Niobate Waveguidesxe2x80x9d now U.S. Pat. No. 6,148,122 .
The present invention relates to synchronous polarization independent all-optical regenerators which include polarization independent modulators for use in, for example, high-speed, long haul, transmissions, and especially in high-speed soliton transmissions.
All-optical regenerators are expected to be a key element in future high-capacity photonic networks since such regenerators provide many advantages compared to their electronic counterparts. The advantages provided by the all-optical regenerators are, for example, bit rate independence, higher speeds, and lower cost. Several types of all-optical regenerators have been proposed in recent years such as, for example, semiconductor optical amplifier (SOA) based regenerators, nonlinear optical loop mirror (NOLM) based regenerators, and synchronous modulation based regenerators.
Essentially unlimited propagation distance at high bit rate ( greater than 10 Gbit/s) has been achieved using the technique of synchronous modulation. In this regard, see, for example, the articles by M. Nakazawa et al. in (a) Electronics Letters, Vol. 27, No. 14, pages 1289-1291, Jul. 4, 1991, entitled xe2x80x9c10 Gbit/s Single-Pass Soliton Transmission Over 1000 kmxe2x80x9d (b) IEEE Journal of Quantum Electronics, Vol. 29, No. 7, pages 2189-2197, July, 1993, entitled xe2x80x9cSoliton Transmission Control In Time And Frequency Domainsxe2x80x9d and (c) Electronics-Letters, Vol. 29, No. 9, pages 729-730, Apr. 29, 1993, entitled xe2x80x9cExperimental Demonstration Of Soliton Data Transmission Over Unlimited Distancesxe2x80x9d. A disadvantage of the synchronous modulators that are used is that polarization dependence is a very detrimental limitation for practical applications, other than possibly integrated high-speed transmitters, since endless polarization tracking would be required.
The article by P. Brindel et al. entitled xe2x80x9c20 Gbit/s Optically Regenerated Transmission over 40 Mm Based on Polarization-independent, Push-pull InP Mach-Zehnder Modulatorxe2x80x9d, in ECOC ""98, pages 685 and 686, September, 1998, discloses a newer type of modulator without polarization dependence. The modulator is a Mach-Zehnder modulator made from InP. However, this type of modulator has a high insertion loss ( greater than 20 dB) which makes it difficult to use in practical applications.
It is desirable to provide an all-optical regenerator which includes polarization independent modulators and advantageously has low insertion loss (e.g.,  less than 8 dB), a low driving voltage, and can be widely used in high-speed transmissions.
The present invention is directed to synchronous polarization independent all-optical regenerators which include polarization independent modulators.
Viewed from one aspect, the present invention is directed to an optical regenerator comprising a first optical path for separately processing a demultiplexed optical channel signal of an optical input signal comprising N wavelength division multiplexed channel signals received by the regenerator. The first optical path comprises clock recovery means and an optical modulator. The clock recovery means is responsive to the demultiplexed optical channel signal for generating an electrical recovered clock signal having a frequency corresponding to a bit rate frequency of the demultiplexed optical channel signal. The electrical recovered clock signal is used in the clock recovery means to generate at least one electrical driving voltage signal. The optical modulator comprises at least three electrodes, and first and second Lithium niobate optical waveguides interspersed between the at least three electrodes. The optical modulator is responsive to both the optical channel signal and the at least one electrical driving voltage signal from the clock recovery means for generating a regenerated optical output channel signal which is polarization independent.
Viewed from another aspect, the present invention is directed to an optical regenerator for use in an optical transmission system comprising a wavelength division demultiplexer (WDD), clock recovery means, optical modulators, and a wavelength division multiplexer (WDM). The WDD is responsive to the reception of an optical input signal comprising N wavelength division multiplexed channel signals for directing each channel signal onto a separate one of N optical output paths. A separate clock recovery means is coupled in each of the N optical output paths of the WDD and is responsive to the optical channel signal directed to the associated optical output path by the WDD for generating an electrical recovered clock signal having a frequency corresponding to a bit rate frequency of the optical channel signal. The electrical recovered clock signal is used by the clock recovery means to generate at least one electrical driving voltage signal. A separate optical modulator is coupled in each of the N optical output paths of the WDD comprising at least two electrodes and at least two Lithium niobate optical waveguides, the optical modulator being responsive to both the associated optical channel signal and the at least one electrical driving voltage signal from the associated clock recovery means for generating a regenerated optical output signal which is polarization independent. The wavelength division multiplexer (WDM) is responsive to the reception of each of the regenerated optical output signals from the optical modulators in the N optical paths for generating an optical output signal. The optical output signal comprises N wavelength division multiplexed channel signals for transmission over a single optical output fiber of the regenerator.
The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims.